Various processes, such as those used in oil refineries, natural gas processing plants and CO2 capture plants, use an amine-based solvent for acid gas absorption from gaseous streams. For example, amine-based solvent may be used for CO2 capture from flue gas of power plant stations, for CO2/H2S absorption from sour natural gas streams or oil refinery tail gas, or for extracting dissolved acid gas species such as H2S or COS from refinery condensate streams such as liquefied petroleum gas (LPG) or natural gasoline. The acid gas absorption process mainly consists of an absorber to remove acid gases from the gaseous stream, and a stripper to regenerate the amine-based solvent by stripping off the absorbed acid gases from the amine-based solvent.
The amine-based solvent may comprise, for example, a primary-, a secondary-, a tertiary-, and/or a sterically hindered amine compound. Exemplary amines may include monoethanolamine (MEA), diglycolamine (DGA), diethylamine (DEA), di-isopropanolamine (DIPA), triethanolamine (TEA), methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP) or a mixture thereof. In addition to the amine compound, an optional co-solvent, such sulfolane (2,3,4,5-tetrahydrothiophene-1,1-dioxide), poly propylene glycol ether, glycerol, or other organic that is soluble in water, can be present in the amine-based solvent.
Amine-based solvents, for example those single or mixed solvents used to absorb the acid gas, may accumulate or form undesired compounds over time which may affect the ability of the amine-based solvent to absorb the acid gas. Undesired waste compounds may include, for example, high-boiling degradation products, ionic species, impurities, fine suspended solids, or any combination thereof. If the undesired compounds are not removed on an ongoing basis, the amine-based solvent may lose a portion of its absorption capacity and operational challenges may occur, resulting in complications with plant operation effectiveness and plant economics.
Operational challenges in a process that uses amine-based solvents for acid gas absorption may include: increased stripping energy consumption; increased corrosion tendency; changed solvent physical properties; fouling of heat exchangers leading to reduced heat transfer efficiency; formation of foaming in the capture plant columns leading to reduced contact between gas and liquid and reduced acid gas removal; loss of solvent effectiveness; increased operating costs due to greater solvent consumption; or any combination thereof.
A reclaimer unit may be used to treat the amine-based solvent in order to separate the amine compound from accumulated undesired compounds in the amine-based solvent. The types of undesired compounds found in the amine-based solvent are a function of the impurities in the gaseous streams, the type of amine compound used, and the operating conditions of the treating unit in an oil refinery, natural gas processing plant or CO2 capture plant. A reclaimer unit may be used, for example, to remove high-boiling degradation products, ionic species, impurities, fine suspended solids from the amine-based solvent, or any combination thereof.
General processes for reclaiming the amine compound from the amine-based solvent include: thermal reclaiming, ion-exchange, electro-dialysis, mechanical filtration, and adsorption (e.g. using activated carbon). These general processes may be used to separate the amine compound from the degradation products, ionic species, impurities, fine suspended solids present in the amine-based solvent, or any combination thereof. Thermal reclaiming is the only process that can remove the majority of degradation products and contaminants in the amine solvent.
Current thermal reclaimers may be complex, expensive and difficult to operate. They may have reduced recovery rates of the amine compound. The amine compound may comprise a large portion of the reclaimer waste. Existing thermal reclaimers operate at high temperatures which can lead to thermal degradation of the amine compound. If excessive water and steam are used to enhance amine compound recovery, the associated amount of energy used may result in commercially untenable costs. Current thermal reclaimers may have long reclaimer operation cycles, where operation cycles corresponds to the operation time required to meet the cleanup target of the solvent. Current thermal reclaimers may use a cold feed stream (for example, from a storage tank or cold lean-solvent stream) for feeding into the reclaimer, resulting in increased energy consumption and increased heating costs associated with the thermal reclamation process.
Examples of known thermal reclaimer processes are disclosed in: U.S. Pat. No. 2,701,750; U.S. Pat. No. 3,664,930; U.S. Pat. No. 4,389,383; U.S. Pat. No. 5,108,551; U.S. Pat. No. 5,137,702; U.S. Pat. No. 5,152,887; U.S. Pat. No. 5,389,208; U.S. Pat. No. 5,441,605; U.S. Pat. No. 5,993,608; U.S. Pat. No. 6,152,994; U.S. Pat. No. 6,245,128; U.S. Pat. No. 6,508,916; U.S. Pat. No. 7,323,600; International Patent Publication WO 93/21148; International Patent Publication WO 98/48920; International Patent Publication WO 99/21821; International Patent Publication WO 00/76624; and U.S. Patent Publication No. 2007/0148068, the disclosures of which are hereby incorporated by reference.
Existing reclaimers for primary alkanolamine solvents, such as MEA and DGA, may be operated at the operational pressure of the stripper, which could be slightly higher than atmospheric pressure, in order to return the reclaimer product vapour to the stripper from the reclaimer unit. Known reclaimers include kettle reboilers, which have a large vapour space at the top of the kettle reboiler. The heat duty is often supplied by high pressure steam using a horizontal U-tube bundle. For secondary and tertiary amines, such as DEA, DIPA and MDEA, existing reclaimers may be operated under reduced pressure with a single batch evaporator or more than one evaporator in serial.
In operation of existing reclaimers, the concentration of high-boiling organic compounds, nonvolatile salts, or both, will increase as the reclaimer is fed with additional amine-based solvent and the amine compound is removed. At operational pressure of the stripper, this increased concentration of high-boiling organic compounds, nonvolatile salts, or both, may result in a sufficient increase in the temperature of the reclaimer so as to increase the decomposition rate of the amine compound being purified, which could result in loss of amine, formation of volatile decomposition products which could contaminate the purified amine product, or both. High salt concentrations in the reclaimer could result in precipitation of solid crystals which could contribute to reclaimer fouling, plugging problems, or both.